136 PALAEONTOLOGY OF ILLINOIS. 



from the obtuse inner angle; postero-lateral border also obliquely 

 and rapidly converging from the subacute posterior extremity towards 

 inrolled margin, basal portion channeled and posteriorly expanding 

 beyond the limits of the rounded, inbeveled coronal fold ; inner mar- 

 gin strongly arched round the base of the posterior prominence, 

 sharply contracted in front, and broadly arched round the anterior 

 lobe to the antero-lateral border. Coronal region divided into nearly 

 equal parts by the relatively narrow, deeply excavated median de- 

 pression, posterior prominence regularly arched transversely into the 

 broad rounded crest, which is interrupted by a few relatively strong, 

 short transverse furrows and corresponding ridges extending over the 

 outer half or more of its extent ; anterior ridge also quite prominent 

 with regular transverse convexity, and similarly marked by trans- 

 verse corrugations. Surface generally minutely punctate. A specimen 

 below medium size measures, in greatest diameter across the inner 

 margin, 13 mm. ; length along antero-lateral border, 3 mm., more 

 or less. 



Other associate forms of the species remain unknown; the small 

 size of the recognized terminal teeth indicate for the unknown forms 

 a diminutive size and delicacy that, in a measure, explains their 

 absence in the collections, which, indeed, contain comparatively few 

 individuals of either of the representative forms described above. The 

 prominent transverse corrugations ornamenting the coronal ridges 

 serve to distinguish the species from its congeners of the subjacent 

 formations and subsequent deposits of the Coal Measures. In the 

 latter respect, also, it presents quite as marked a contrast with the 

 European species, originally distinguished under the names, Pcecilodus 

 Jonesii and Pcecilodus obliquus, Ag. 



Geological position and locality : Chester limestone ; Chester, Illinois. 

 PCECILODUS WORTHENI, St. J. 



PI. VIII. Fig. 18. 



The Chester formation has recently afforded an example of a man- 

 dibular terminal tooth of Poscilodus. This is referable to a species 

 markedly distinct from all others known to me. The specimen was 

 obtained, probably, from the limestone overlying the horizon from 

 which the majority of the fish-remains of the Chester were derived, 

 and is, thus far, a unique representative of its kind. The tooth 

 belongs to the left ramus, and, besides its great size, it is dis- 

 tinguished by the following characteristics: 



